Methods and devices for the dynamic testing of materials

ABSTRACT

A conical housing is provided with a stick of explosive disposed axially within the housing. The apex angle of the housing is chosen so that a shock wave front produced by igniting the explosive is tangential to an internal conical wall of the housing. A plurality of measurement sites are disposed over the internal wall and specimens to be tested are mounted therein. The stick of explosive is ignited and the resulting distortion of the specimens is observed.

United States Patent [72] lnventors Erhard Hornbogen Gottingen; Erwin Niessner, Neuenburg, Baden; Gert Buck, Bad Uberkingen, Germany [21} Appl. No. 811,053 [22] Filed Mar. 27, 1969 [45] Patented May 4, 1971 4 [73] Assignee Buck K. G.

Bad Uberkingen, Germany [54] METHODS AND DEVICES FOR THE DYNAMIC TESTING'OF MATERIALS 7 Claims, 11 Drawing Figs.

[52] US. Cl I 73/12, 7 3/ 35 [51] Int. Cl G01n 3/08 [50] Field ot'Search 73/12, 35, 147, 87, 88

[56] References Cited UNITED STATES PATENTS Re26,'279 10/ 1 967 Boynton 73/ l 2 2,761,309 9/1956 Pearson et al. 73/35X 3,184,955 5/1965 Filler 73/35 3,270,556 9/1966 Henriksen 73/95 3,277,693 10/1966 DAmato et al. 73/95X OTHER REFERENCES NAVORD REPORT NO. 6844, Design Characteristics of a Conical Shock Tube for the Simulation of Very Large Charge Blasts", July 26, 1960, pp. 1 l5, copy in 73-12.

Primary Examiner-Charles A. Ruehl AttorneyBurns, Doane, Swecker and Mathis ABSTRACT: A conical housing is provided with a stick of explosive disposed axially within the housing. The apex angle of the housing is chosen so that a shock wave front produced by igniting the explosive is tangential to an internal conical wall of the housing. A plurality of measurement sites are disposed over the internal wall and specimens to be tested are mounted therein. The stick of explosive is ignited and the resulting distortion of the specimens is observed.

Patented May 4, 1971 4 Sheets-Sheet 1 VII I14 Ill 1 Patentcd Ma 4,1971 {3,577,762

4 Sheets-Sheet 2 Patente ed May 4,1971

I 4 Sheets-Sheet 4 METHODS AND DEVICES FOR THE DYNAMIC TESTING OF MATERIALS The present invention relates to a method of and a device for the dynamic testing of materials by an explosive charge.

In order to assess the properties of materials, a whole series of test procedures are known which produce quantitative figures for strength, ductility, elastic limit, impact resistance and so on. These test procedures are based upon static test loads, with the exception, that is, of the impact test (Izod test on notched specimens) where although dynamic forces are involved, the energies used are relatively low.

These test procedures, however, do not satisfy the purposes of investigation of materials which, during processing, are exposed for very short times to extremely high loadings, for example materials which are shaped, hardened, plated or, in powder form, compacted, using explosion-produced shock 'waves, in the application of extremely high-strength magnetic fields or the discharge of capacitor banks; equally, these test procedures do not satisfy the requirement in respect of materials which, for example like armor plate, are intended for the manufacture of components which are to withstand ultrahigh loadings. Materials of this kind are therefore subjected to what are referred to as explosion tests, in which explosive charges are I ignited in, for example, for example, tubular test specimens, the explosion toughness" being determined by measurements of the distortion. These dynamic impact tests using explosive charges, produce high rates of loading in a relatively simple way, and make it possible under test conditions to very closely simulate the actual processing conditions, for example, as encountered in using explosion shock waves for-metal-forming.

The known explosion tests, however, are open to a series of drawbacks. For example, in order to obtain a wide range of tests conditions and therefore a wide range of curves showing the way in which the material properties depend upon the loading, a plurality of individual explosions must be used, and this is not only an expensive and time-wasting business but, in particular, presumes that explosive charges with precisely staged explosion strengths are available. However, as those skilled in theart will readily appreciate, it is already difficult enough to produce exactly identical test charges, and the production of test charges of precisely staged explosive strengths, is even more difficult since the explosive strength depends both upon the nature of the explosive used and also upon the quantity of explosive, the physical dimensions and the shape of the charge. For this reason, it is virtually out of the question in conventional explosive tests, to obtain precise technological series of data, in the manner which is achieved in the conventional test processes, i.e. data which will give an indication as to the way in which the properties of a material depend upon different loadings and which are so reproducible in nature that different materials can be compared with one another and a comparison made with the results of other methods of testing applied to the same material. Finally, the

- known explosive tests are limited to a small number of tests shapes, indeed more or less to tubular specimens, the manufacture of these, particularly where boring operations involving hard steels are concerned, being extremely difiicult.

The object of the invention, therefore, is a method and a device for the dynamic testing of materials of all kinds by explosive charges, which method and device enable a wide range of test conditions to be produced, yield accurate and easily reproducible results, and are suitable for the measurement of a plurality of easily manufactured test shapes.

In accordance with the invention, this object is achieved by means of a method which resides in the fact that several identical test specimens of the material being investigated, are distributed along a stick of explosive, in each case of such varying intervals from the stick that when the stick is ignited at one end, the detonation shock wave simultaneously reaches all the test specimens. A preferred device in accordance with the invention, for carrying out this method,'is characterized by a conical housing in which a cylindrical stick of explosive is inserted in an axial attitude, and which has an apex angle such that the internal wall of the housing is disposed tangentially to the shock wave front produced with ignition of the stick of explosive, and by measurement sites distributed over the internal wall of the housing, each site consisting of a mounting for a test specimen and of an aperture in the housing wall with the specimen being operably positioned relative to the aperture so as to be subjected to dynamic testing force generated by the shock wave directed from within the housing toward the aperture.

In accordance with the invention, therefore, several identical test specimens are exposed to the shock wave produced by a single explosive charge. By virtue of the fact that the test specimens have different distances from the exploding charge, they are exposed to different loadings and in this way it is possible to determine the way in which the properties of a specific material depend upon loading level. With this kind of explosive charge, however, comparable results are only actually achieved on several test specimens, if it is ensured that all the specimens are subjected in an identical way and without mutual influencing, to the effect of the shock wave. In accordance with the invention, therefore, by suing a stick of explosive and suitably distributing the test specimens along the stick, it is ensured that all the specimens are simultaneously encountered by the shock wave front, so that random effects of reflections and so on are excluded. The use of sticks of explosives also presents a number of advantages because explosive charges of this kind are still the easiest to manufacture to precise spatial dimensions. It should be pointed out in this context that the term stick used for convenience, is intended to cover elongated bodies of the most varied geometric dimensions, for example strips of explosive and also thin foils of explosive.

The device in accordance with the invention enables the said method to be implemented particularly easily, because the housing design has been determined, in particular the apex angle, then depending upon the explosive charges used the process conditions which apply to the measurement sites located on the internal wall of the housing, will be maintained throughout. Furthermore, the attachment of the specimens to the test sites presents no difficulty at all and by suitably designing the test specimens the particular relevant measurement data can be obtained in a manner which will be described in detail hereinafter.

Other features, advantages and details of the invention will become apparent from the description, drawing and claims.

DRAWING OF PREFERRED EMBODIMENT In the drawing, a preferred embodiment of the invention has been illustrated by way of example and in fact:

FIGS. la, lb, 1c are diagrams illustrating the method of the invention;

FIG. 2 is a device in accordance with the invention viewed in longitudinal section;

FIG. 3 is a plan view of the device of FIG. 2;

FIG. 4 is a cross section on the line 4-4 of FIG. 2;

4a is a cross section of the housing of FIG. 2 taken along line 4-4 of FIG. 2 and showing a test specimen connected to carry out tensile tests,

FIG. 5 is a test specimen in plan;

FIGS. 60 and 6b are diagrams of test results obtained by the application of the invention; and

FIG. 7 is an illustration of test specimens after the explosive test.

DETAILED DESCRIPTION The fundamental principle of the invention resides in atranging several identical specimens of the material being investigated, at varying intervals from an explosive charge in order, using a single such charge, to be able to carry out tests at different loadings. In order that the shock wave produced by the explosion shall reach all the test specimens simultaneously, an explosive charge of stick form is used, this being ignited at one end and all the specimens being arranged at locations which are simultaneously reached by the shock wave front. This fundamental principle of the invention will now be explained in more detail making reference to FIG. 1. In FIG. la, a cylindrical stick of explosive of length L is shown arranged on the y-axis of a coordinate system, this in such fashion that the stick extends from the Y=O to the point Y=L. If this stick 10 is ignited at its top end, that is to say, the location Yi, then a shock wave is developed. If, simplifying matters, we first of all assume that the rate of ignition V in the stick 10 is constant and the rate of propagation V, of the shock wave produced by the explosion, is likewise constant and equivalent to the speed of sound, then we obtain a wave front of the kind shown in FIG. lfor three different elapsed times after ignition of the explosive charge. The apertural angle a of the conical wave front is here given by tan (1:2 Y

However, this means that all the test specimens which lie for example on the conical surface marked 11, the latter having an apex angle a and being tangential to the wave front, will be reached by the shock wave at the same time, independently of what distance x they have from the stick 10 or explosive. On the other hand, however, the strength of the shock wave to which the test specimens are subjected, depends upon the distance x from the stick of explosive since the pressure P, is a function of the distance x. If, therefore, for example a first specimen is located at the point P um a second at the point P (X ay both points being on the conical surface 11, then although both specimens will be simultaneously reached by the shock wave, the loadings they experience will be different.

The above assumption, namely that the speed of propagation V I is constant, is purely an approximation. Strictly speaking, V, is dependent upon the pressure P,, the latter in turn, as already mentioned, falling off with increasing distance x. Accurate measurements therefore shown a wave front of the kind illustrated in FIG. 10. In order therefore to achieve precise simultaneity in the times of arrival of the shock wave at the various specimens, the latter have to be arranged upon a curved conical surface, however, for the degree of accuracy required in practice the straight conical surface 11 will suffice, its apex angle a being corrected however to accord with the pressure dependence of the propagation speed V Summarizing once again, therefore, the method of the invention resides in arranging several specimens at different points on the conical surface 11 and in this way exposing them simultaneously to different loadings produced by a single explosive charge. The simultaneity of the loading also achieves the advantage that the influence of inevitable reflection waves is excluded.

In FIGS. 2, 3 and 4 a device for implementing the method of the invention has been illustrated. The device consists essentially of a funnel-shaped housing 12 made up of two half shells plus a first clamping ring 13 serving as base and clamping ring 13b placed over the top rim of the funnel and containing holes accommodating dowels on the funnel shells. The base section of the funnel-shaped housing 12 includes an insert tube 14 in which the bottom end of the cylindrical stick 10 of explosive is inserted. A centering cross member 15 located on the top edge of the funnel-shaped housing 12, maintains the stick 10 of explosive in the axial position. At the free end of the stick of explosive 10, this projecting out of the housing 12 to do duty as a fuse, there is fitted a detonator 17, a relay 16 being interposed, which detonator can be electrically exploded through the leads 18. On the wall of the housing 12, measurement sites are provided; the drawing actually shows seven measurement sites arranged in a spiral line. Each site consists of a wall hole 19 with a vertical backing wall, in which threaded holes are provided for screws, the vertical backing wall being broken right through to the outside in the form of a circular hole 20. Each hole or recess 19 is designed to take a test specimen 21 which is fixed in position there by screwing it to the backing wall so that the center portion of the test specimen 21 masks over the hole 20. The platelike test specimens 21 are intended for bending tests and have the form shown in FIG. 5.

The hole 20 may be clear so as to subject the specimen 21 to only a direct blast or, alternatively, a ramlike piston 22 may be slidingly disposed within the hole 20 and may be attached to the specimen 21 so as to deform the specimen 21 when the blast pressure acts on an exposed surface of the piston 22. The practicing of the present invention with a ram mass such as the piston 22 is discussed in detail later in the specification.

The internal conical surface of the housing 12 is designed in accordance with the basic principles explained in relation to FIG. 1. In other words, the apex angle of the conical surface defined by the internal wall of the housing is such that said wall is disposed tangentially to the front of the shock wave produced with ignition of the explosive charge 10.

In order to carry out the explosive test, a stick 10 of explosive is ignited at its top end through the medium of the lead 18, the detonator 17 and the relay l6, whereupon the downwardpropagating explosion generates a shock wave front which reaches all the seven measurement sites and therefore all the seven test specimens 21, simultaneously. The test specimens are deflected at their central regions where they cover the holes 20, the magnitude of the deflection being the greater the closer the particular specimen is to the stick 10 of explosive. In other words, the topmost specimen, near the rim of the housing, is most lightly loaded whilst the bottommost one, near the base of the housing, receives the heaviest loading.

After the explosion has taken place, the clamping ring 13b is removed and the housing 12 released from the clamping ring 13, whereupon the individual test specimens can be removed from the two open half shells of the housing. The distortion of the individual specimens is measured and these figures then indicate how the bending properties of the material used depend upon the strength of the explosive loading.

In a practical test designed to investigate bending properties, seven measurement sites were used and these in fact had the following intervals from the center axis of the housing 12 and therefore the stick 10 of explosive:

Measurement Distance from site No: center x (mm.)

Making the simplified assumption that the pressure P, is inversely proportional to the distance x therefore, between the measurement sites 1 and 7 the dynamic loading can be changed by a factor around 6. A higher factor would simply mean larger dimensions on the part of the housing and a longer length on the part of the stick 10 of explosive. Another possible way of increasing the range of measurement resides in using several explosions of differing strengths. It should be borne in mind in this context that when using sticks of explosive of substantially differing dimensions, or when using widely different explosives, correspondingly adapted funnelshaped housings must be provided. In practice, it has been found that for the testing of virtually all commercial materials, with the kind of samples shown in FIG. 5 three different explosive charges provide an adequately wide range of measurement.

The pressure P at the internal surface of the housing 12 can obviously be employed not only to carry out bending tests but A comparison with FIG. 6b shows that the harder steel (S170), despite its smaller extension or strainin the-static tensile test, exhibits ,good deformability, characteristics" in the dynamic test. The suggests that the data obtained by the conyentional tensile tests, do not necessarilyprovidea good in- "dicator to the behavior which the materiaLwouldexhibit ported area) of 16 (4X4), 5X5) and 50 5XIQ) mm. The v thickness of thespe'cimens was l mm. in allcases. For the bending test, the particular loading and deflection has to be related to theload bearing cross-sectional area, whilst for tensile tests it'ha s to be'related to the loading'area. The plastic deformation, on the other hand, can be determined by apply ing indicator marks to the specimens or simply by measuring the deflection. The top limit of plastic deformation can in this context be regarded as the occurrenceof semicircular deflection (depression) of a specimen,.since with any further deformation the shape of the specimen ceases tobe determinate and rupture occurs. However, generally rupture takes place at much lower deformations than this. On all these tests, however, it must be ensured that the most uniform' possible loading is exerted upon the actual measurement area of the specimen, whilstas far as possible all the' other areas of the specimen remain unloaded. I

FIG. 6a illustrates a diagram of the test results obtained in determining the ductility of deep-drawing steel (St27). Here, the ordinates plot the reciprocal of the distanceR between the measurement sitesand the longitudinal axis of the stick of explosive, whilstthe abscissae (x-axis plot the values of the particular deflections of the specimens. Three parallel tests using the same explosive charges and specimens, werecarried out. The spread in the FIGS. amounts to :10 percent. An accuracy of this order can therefore equally be assumed when comparing different materials using the same explosive charge throughout. The origin of the curve (intersection) on the yaxis corresponds to the yield'point, but the terminal point (failure of the specimen) has not been shown. Where the obtaining of precise absolute figures in the load-extension dia gram is concerned, it is obviously necessary to know the relationship P (l/R.) In many cases, however, a knowledge of the relative extension characteristic will suffice. The second curve B in FIG. 60, corresponds with a test carried out using a stronger explosive charge which, as one might expect, gives higher extension figures. In order to be able to obtain the same loading scale, the scale of the y-axis must be correspondingly modified. It has been found that the extension figures at lower pressures correspond very well with one another, but do not always give satisfactory results at higher pressures. Thus, under certain conditions, results obtained using different explosive charges, maybe compared with one another.

In FIG. 6b, the-results of a further series of tests have been plotted. Here, a hardenable carbon steel (S170) and commercial pure aluminum have been compared with the test results of FIG. 6a. Also, a test was carried out on a brittle organic synthetic material (Pertinax). Qualitatively, the results coincide with the behavior that the static-mechanical properties might lead one to expect. The aluminum ruptures beyond measurement site 5 and even at small values of HR exhibits very large extension. The extension of the steel having the higher strength, is smaller as might be expected. It is interesting to note that the ratio of the extensions of the two steels is virtually constant at 2:3 for all the pressures. In all the synthetic material specimens, on the other hand, rupture takes place so that no extension curve could be plotted.

If we now comparethe results obtained with those of the conventional static test methods,the substantial differences will be observed. The two steels (S127 and St70) tested, give the followingresults under static tensiletesting conditions:

Yield point 16 and 65 kplmm Tensile strength 29 and 70 kp/mm.

Braking strain 63 percent and 3 percent.

under extremely high dynamic loading..A systematic comparison of the results obtained with the two test methods, of

matter of extreme importance.

. rnaterialsof the most widely varying properties, is, therefore a fiction test illustrated, constitutes onlyone possible. application. of ,the method of the invention. By incorporating additional ancillary devices for locating the test specimens, dynamictensile,compressive and powder compacting tests can be carried out. In the-tensile test, specimensZl are secured by one end 21a to the funnel-shaped As already mentioned, thefdefl housing 12, whilst theother end 21b carries a ram device 22 which is accelerated at the particular measuring site by the pressure P,. The acceleration and therefore the deformation rateldepend up0n;the pressure P, and the mass of the ram device, these factors having tobe constant, in order to obtain comparable results. The stress in the specimen can be set by giving it a suitable cross-sectional area. Where the compressive and powder-compacting tests are concerned, the back of the specimens are on contact with the specimen mount, and the accelerated ram device compresses the specimen. The mountings can be so designed that at the measurement sites arbitrary tests of thesaid kind can be carried out. Also, it is possible to provide inthe lower partof' the funnel-shaped housing a device for carrying out tests onspecimens in direct contact with the explosive (for measurementsassociated with explosive hardening).

Yet again, it is possible to produce the explosions in a medium other than air, for example in water. In many instances (consider for example steel of the kind used in submarine construction), this maybe an advantage because the pressure drop in water is higher than in air so that a wider measurement range is obtained. Finally, tests can also be carried out at different temperatures provided that the explosive itself introduces no limitation in this direction. I

Where the invention is concerned, the explosive used is of vital importance. In accordance with FIG. 2, the stick 10 of explosive is ignited by a detonator 17. Because the full speed of detonation must be achieved as early as the level of the first measurement site, it is essential to arrange for a corresponding fuse or leader section. This fuse or leader section will depend primarily uponthe nature of the explosive and upon its diameter. In order to apply uniform loading to all the test specimens, it is also necessary for the explosive to have the same diameter throughout its length and in particular for it to be uniform and free of any cracks. The best results have been obtained using a plastic explosive of approximately the following composition:

Nitrocellulose 33 percent Nitroglycerine 28 percent Nit'iopenta 33 percent a Stabilizers and Gelatinizers 6 percent The manufacture of this explosive is carried out in the plasticized hot condition and it can be produced in seamless form, in arbitrary-lengths and diameters, by extrusion. No unm ing p n mena occur in the gelatinous condition. Furthermore, this explosive has the advantage that, as trials have confirmed,'the-shock wave speed of about 800 m./sec.' is virtually independent of the diameter of thestick of explosive. This c. igniting the stick at one end thereof.

2. A device't'or dynamically testing materials comprising:

a. a conical housing;

b. a cylindrical stick of explosive disposed axially within said conical housing;

c. said conical housing having an apex angle formed by an internal wall thereof;

d. said apex angle being of such degree that a shock wave front produced by igniting said stick of explosive is tangential to said internal wall;

e. a plurality of measurement sites formed in and distributed over the surface of said internal wall; and

f. said measurement sites each comprising;

i. a portion of said internal wall formed to define a hole,

ii. a mounting on said wall for securing a specimen to be tested thereto with the specimen being operably positioned relative to said hole so as to be subjected to dynamic testing force generated by said shock wave directed from within said housing toward said hole.

3. A device according to claim 2 wherein said conical housing comprises:

a. two half shells;

sites formed in said internal wall are disposed on a spiral line.

6. A device according to claim 2 with the addition of:

a. slidably mounted pistons;

b. said pistons disposed within said holes;

c. said pistons operable to act upon the specimens when said stick of explosive is ignited.

7. A device according to claim 2 wherein said explosive stick comprises a plastic explosive. 

1. A method for dynamically testing materials comprising the steps: a. providing a stick of explosive; b. distributing a plurality of identical test specimens of a material to be tested along the length of the stick and at varying distances from the stick whereby a detonation shock wave simultaneously reaches all the specimens when the stick is ignited; and c. igniting the stick at one end thereof.
 2. A device for dynamically testing materials comprising: a. a conical housing; b. a cylindrical stick of explosive disposed axially within said conical housing; c. said conical housing having an apex angle formed by an internal wall thereof; d. said apex angle being of such degree that a shock wave front produced by igniting said stick of explosive is tangential to said internal wall; e. a plurality of measurement sites formed in and distributed over the surface of said internal wall; and f. said measurement sites each comprising; i. a portion of said internal wall formed to define a hole, ii. a mounting on said wall for securing a specimen to be tested thereto with the specimen being operably positioned relative to said hole so as to be subjected to dynamic testing force generated by said shock wave directed from within said housing toward said hole.
 3. A device according to claim 2 wherein said conical housing comprises: a. two half shells; b. a first clamping ring; c. a second clamping ring; d. said first clamping ring forming a base for said housing; e. said second clamping ring being fitted to a rim of said housing; f. said first and second clamping ring operable to hold said two half shells together to form said conical housing.
 4. A device according to claim 2 with the addition of: a. a centering crossmembers; and b. said Centering crossmember operative to locate said stick of explosive axially within said conical housing.
 5. A device according to claim 2 wherein said measurement sites formed in said internal wall are disposed on a spiral line.
 6. A device according to claim 2 with the addition of: a. slidably mounted pistons; b. said pistons disposed within said holes; c. said pistons operable to act upon the specimens when said stick of explosive is ignited.
 7. A device according to claim 2 wherein said explosive stick comprises a plastic explosive. 